Power supply lines in a semiconductor integrated circuit (IC) supply current to charge and discharge active and passive devices in the IC. For example, digital complementary metal-oxide-semiconductor (CMOS) circuits draw current when the clock makes a transition. During the operation of circuits, the power supply lines supply transient currents with a relatively high intensity, which can result in voltage noise on the power supply lines. The voltage on the power supply line will fluctuate when the fluctuation time of the transient current is short or when its parasitic inductance or parasitic resistance is large.
The operational frequency of the IC may be on the order of several hundreds of mega-hertz (MHz) to several giga-hertz (GHz). In such circuits, the rising time of clock signals is very short, so that voltage fluctuations in the supply line may be very large. Undesired voltage fluctuations in the power supply line powering a circuit can cause noise on its internal signals and degrade noise margins. The degradation of noise margins can reduce circuit reliability or even cause circuit malfunction.
To reduce the magnitude of voltage fluctuations in the power supply lines, filtering or decoupling capacitors are usually used between the terminals of different power supply lines or between terminals of power supply line and the ground line. Decoupling capacitors act as charge reservoirs that additionally supply currents to circuits to prevent momentary drops in supply voltage.